This project focuses on cognitive function and cognitive change in older adults with usual aging, the intermediate states that precede dementia and diagnosable AD. The primary objective of this project is to distinguish between normative and pathological cognitive aging by modeling intraindividual cognitive variability and change in older adults. Recent research has demonstrated substantial intraindividual cognitive variability in older adults and that high levels of variability may signify underlying neurological deficits, such a s Alzheimer's disease or the intermediate states that precede dementia. Two measurement-intensive longitudinal studies are proposed that will extend knowledge about the relationship among performance variability and cognitive impairment in aging in several important ways. The first study is a measurement burst design that will examine daily variations in cognitive function in persons who are cognitively normal and in those with Amnestic Cognitive Impairment (ACI). The second study is a longitudinal study that will focus on moment-to-moment performance fluctuations. The proposed research has three overall aims: 1) To test the hypothesis that daily fluctuations of stress and physical symptoms predict day-to-day variability in cognition. 2) To determine whether high levels of cognitive variability and slowed retrieval speed signify elevated risk for developing dementia. 3) To determine how to use information about level, speed, and variability in cognitive performance to optimize classification and early detection of cognitive impairment associated with early Alzheimer's Disease (AD). The results will provide information needed to detect preclinical and clinical AD, discriminate AD-related cognitive changes from age-related cognitive changes, investigate cognitive aging without dementia, provide early treatment of AD, and correlate cognitive physiologic, biologic, neuropathologic and neuroimaging changes in aging and AD.